This invention relates to nuclear magnetic resonance (NMR) imaging.
NMR imaging techniques can be used to form a picture of a cross-section of three-dimensional objects (for example, human organs) in which their structure is indicated by variations in intensity or color of the picture.
One common technique for forming such an image uses a first magnetic field pulse having a linear gradient along a z-axis (G.sub.z) to select the "slice" corresponding to the desired cross-section, a second pulse (called a phase-encoding pulse) having a linear gradient along the y-axis (G.sub.y) to encode nuclei at different y-axis positions with different precessional phases, and a third linear gradient pulse (called a frequency-encoding pulse) along the x-axis (G.sub.x) to encode nuclei at different x-axis positions with different frequencies. An appropriately modulated RF signal generator imposes a 90.degree. RF pulse followed (after an appropriate interval) by a 180.degree. pulse. The resulting time-dependent resonance spin-echo signal is measured and stored. The process is repeated to obtain a family of spin-echo signals each based upon a different magnitude of phase-encoding gradient G.sub.y. The family represents a two-dimensional array of time-dependent information. A two-dimensional Fourier transformation of the spin-echo signal array produces a two-dimensional array of frequency-domain data which can be displayed as an image of the selected slice.
In addition to such images of the structure of organs, it has been suggested that NMR techniques be used in analyzing flow characteristics. For example, information about the flow of blood in an artery could be useful in analyzing deformities of the wall of the artery.
Moran, "A Flow Velocity Zeugmatographic Interlace for NMR Imaging in Humans", Magnetic Resonance Imaging, 1983, discloses adding to the usual imaging gradients, a special sequence of gradient pulses (for example, along the z-axis) to encode the nuclei with information about their velocity which can then be recaptured by Fourier transformation. The special gradient pulses are arranged to eliminate any dependence of the velocity-encoded information on spatial location. The article suggests using the imaginary component of the resulting data as an image of flow-current-density, and the ratios of the real to imaginary components of the data as an image of specific-flow-density.
In addition to cross-sectional images, NMR techniques have been used to produce three-dimensional projection images in which the data for a stack of cross-sectional slices are effectively added together. Macovski, "Selective Projective Imaging: Applications to Radiography and NMR," IEEE Transactions on Medical Imaging, July, 1972, discloses selective projective imaging in which subtraction of unwanted image components is proposed to be used in displaying moving blood without displaying the surrounding tissue.
Hahn, E. L., J. Geophys. Res. 65, 1960, p. 776, recognized that the motion of nuclei in a magnetic gradient will modify their phases in a way which is reflected in a shift in phase at the center of the spin echo.